1. Field of the Invention
The present invention relates to a process for continuously forming a large area functional deposited film by sustaining a substantially uniform microwave plasma over a large area to cause plasma reactions by which a film-forming raw material gas is excited and decomposed, and to an apparatus suitable for practicing said process. More particularly, the present invention relates to a process for continuously forming a large area functional deposited film of uniform thickness with a markedly improved gas utilization efficiency of a film-forming raw material gas and at a high deposition rate by microwave PCVD method and to an apparatus suitable for practicing said process. The process and the apparatus enable one to mass-produce large area thin film semiconductor devices such as photovoltaic devices at a reduced cost.
2. Background of the Invention
Along with a marked increase of power generation in order to meet an increased demand for a means of power supply in recent years, problems of environmental pollution have become serious in the world.
In fact, for the atomic power generation system which has been anticipated as a power generation system capable or replacing the steam-power generation system and which has been operated in some places of the world, there occurred events in which the systems were broken down to cause radioactive contamination of living things including human. Because of this, there is a fear for further development of the atomic power generation system and there are some countries that already prohibit to newly establish an atomic power plant.
Now, in the case of the steam-power generation, the amount of a fossil fuel represented by coal or petroleum to be consumed for power generation in order to comply with a societal demand for increased power supply has been continuously increased and along with this, the amount of exhaust fumes from the steam-power generation plants has been continuously increased accordingly to raise the content of gases to cause a greenhouse effect such as carbon dioxide gas in the air. This results in providing an earth-warming phenomenon. In fact, the annual average temperature of the earth has been increased in recent years. In order to prevent said earth-warming phenomenon from further developing, the international Energy Agency has proposed to reduce the amount of carbon dioxide to be exhausted from the steam-power generation plant as much as 20% of the current level by the year 2005.
Against this background, there is a situation that the populations of developing countries will continue to increase and along with this, demand for power supply will be further increased. In addition to this, it is expected that the manner of living in the developed countries will be further modernized with further developments in electronic instruments and along with this, the amount of power consumption per person will be eventually increased.
In view of the above, the matter of power supply is now the subject to be internationally discussed in terms of the earth.
Under this circumstance, public attention has now focused on and various studies have been made on the power generation system using a solar cell since it has various advantages: it is a clean power generation system which is free of the foregoing problems relating to the radioactive contamination, earth-warming and environmental pollution; the sunlight to be used as its energy source reaches everywhere on the earth and there is not a problem that the energy source be localized; and the power generation equipment can be simplified and a relatively high power generation efficiency can be attained.
Now, in order for the solar cell power generation system to be made such that it can supply power in a quantity to satisfy the social demand, it is basically required that the solar cell to be used provide sufficiently high photoelectric conversion efficiency, it can stably exhibit solar cell characteristics and it can be mass-produced.
In order to provide the average family with the power to be consumed, a solar cell capable of outputting a power of about 3 KW is necessary. In this case, the photoelectric conversion efficiency of the solar cell should be about 10%. The solar cell is required to have an area of about 30 m.sup.2 in order to provide said power. In the case where it is intended to satisfy the demands for power supply for 100,000 families, the solar cell is required to have an area of 3,000,000 m.sup.2.
In view of this, the public attention has been focused on an amorphous silicon solar cell which is prepared by depositing a semiconductor film such as an amorphous silicon semiconductor film on a relatively inexpensive substrate such as glass or metal sheet wherein glow discharge is caused in a film-forming raw material gas such as silane gas, with the viewpoint that it can be mass-produced and it can be provided at a lower cost in comparison with a single crystal silicon solar cell. Various proposals have been made already on the amorphous silicon solar cells.
In the case of the power generation system using a solar cell, there is usually employed a system in which a plurality of unit modules are connected in series or in a row to be a unit from which a desired current or voltage can be obtained. For each of the plurality of modules, it is required that neither disconnection nor short circuits occur. It is further required that each of said plurality modules stably outputs an even current or voltage. In order to satisfy these requirements, it is necessary to prepare each unit such that its constituent semiconductor layer as a most important element be ensured to stably exhibit uniform characteristics required thereof.
Further, from the viewpoint of making it easy to design the module and also from the viewpoint of simplifying the process for assembling a plurality of unit modules into a unit, it is essential to provide a large area semiconductor film having uniformity not only in the thickness but also in quality and which is capable of exhibiting uniform semiconductor characteristics. These lead to enabling the mass-production of a solar cell and to extreme reduction in the production cost.
Now, in the solar cell, its constituent semiconductor layers, which are basically important constituent elements thereof, are conjugated to form semiconductor junctions, such as pn junctions or pin junctions. These semiconductor junctions can be attained by stacking different semiconductor layers respectively having a different conduction type one from another, or by ion-implanting or thermally diffusing a dopant of a different conduction type into one of the constituent semiconductor layers of the same conduction type.
This situation will be described in more detail in the case of the foregoing amorphous silicon solar cell. It is known that glow discharge is caused in a gas mixture composed of a film-forming raw material gas such as silane gas (SiH.sub.4) and a raw material gas capable of supplying an element to be a dopant such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) to form a semiconductor film having a desired conduction type. When a plurality of semiconductor films respectively having a different conduction type are formed successively on a substrate in this manner, these semiconductor films are conjugated to form desired semiconductor junctions. In view of this, there have been made various proposals that respective constituent semiconductor layers are separately formed in the respective independent film-forming chambers to stack them on a substrate to form a desired semiconductor junction between each pair of the semiconductor layers stacked, whereby obtaining an amorphous silicon solar cell.
For instance, the specification of U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD apparatus wherein a so-called roll-to-roll system is employed. The continuous plasma CVD apparatus comprises a plurality of RF glow discharge regions through each of which regions a substrate web on which a film is to be formed is moved. The specification describes that said apparatus makes it possible to prepare an element having one or more semi-conductor junctions by forming a semiconductor film of a desired conduction type on said substrate web in each of said plurality of RF glow discharge regions while moving said substrate web. The specification describes that a gas gate is provided between the adjacent glow discharge regions in order to prevent a raw material gas used in one glow discharge region from entering into other glow discharge regions. In more detail in this respect, said plurality of glow discharge regions are isolated one from the other by an isolation passageway provided with means for forming a cleaning gas stream of Argon, H.sub.2, etc. It can be said that this roll-to-roll plasma CVD apparatus will be suitable for the mass-production of a semiconductor device. However, this roll-to-roll plasma CVD apparatus is problematic in the case of mass-producing a semiconductor device with a plurality of semiconductor junctions in that, since each of the constituent semiconductor layers is formed by the plasma CVD method using RF energy, there is a limit for continuously forming those constituent semiconductor layers at a high deposition rate while maintaining the characteristics desired for each of those constituent semiconductor layers. That is, even in the case of forming a thin semiconductor layer of, for example, about 5000 .ANG., it is necessary to always sustain a substantially uniform plasma over a large area. However in this roll-to-roll plasma CVD apparatus, there are many correlated film-forming parameters which are difficult to be generalized and well-skilled technicians are required to do so. In addition to this, there are still other unresolved problems for the roll-to-roll plasma CVD apparatus in that the decomposition rate and the utilization efficiency of a film-forming raw material gas are not sufficient and thus the product becomes unavoidably costly.
Japanese Unexamined Patent Publication Sho. 61(1986)-288074 discloses a roll-to-roll film-forming apparatus comprising a reaction chamber containing a hood-like shaped curtaining portion of a flexible substrate web which is delivered by a pay-out mechanism and taken up by a take-up mechanism, said reaction chamber having a reaction space circumscribed by said hood-like shaped curtaining portions, and said reaction chamber being provided with at least an activation chamber isolated from said reaction chamber. The film formation by this apparatus is carried out by introducing active species formed in said activation chamber and if necessary, other film-forming raw material gas into said reaction space, wherein they are chemically reacted by the action of a heat energy to form a deposited film on the inner surface of said hood-like shaped curtaining portion positioned in said reaction chamber. This roll-to-roll film-forming apparatus is advantageous from the viewpoint that the apparatus can be relatively compact and the deposition rate of a film to be formed may be improved because of using an active species in comparison with the known plasma CVD apparatus.
The film formation by this roll-to-roll film-forming apparatus utilizes the chemical reaction to cause film formation with the aid of heat energy. Therefore, when the deposition rate of film to be formed is desired to be increased, it is necessary to increase not only the flow rate of an active species introduced into the reaction space but also the quantity of heat energy to be supplied thereinto. However, it is extremely difficult to do so since there is a limit not only in the manner of generating a large amount of the active species in the activation chamber and sufficiently introducing the active species into the reaction space at a high flow rate without leakage but also for uniformly supplying a large quantity of the heat energy into the reaction space.
In recent years, a plasma CVD method using microwave glow discharge decomposition, namely, a microwave plasma CVD method (which will be hereinafter referred to as "MW-PCVD method") has been noticed on the industrial scale since the MW-PCVD method has various advantages, which cannot be attained by the RF glow discharge decomposition method, that it is possible to heighten the energy density, to effectively generate a plasma and to maintain the plasma in a desired state.
For instance, the specifications of U.S. Pat. Nos. 4,504,518 and 4,517,223 describe processes for forming deposited thin films on small area substrates in a microwave glow discharge plasma under a low pressure condition. These two patent specifications describe that because the processes are conducted under the low pressure condition, any of these processes makes it possible to obtain a high quality deposited film at a remarkably high deposition rate while eliminating not only polymerization of active species which gives negative effects to the characteristics of a film to be formed but also formation of powder such as polysilane in the plasma. However, neither of these two patent specifications mentions anything about uniform deposition of a film over large area.
The specification of U.S. Pat. No. 4,729,341 discloses a low pressure microwave plasma CVD method and an apparatus suitable for practicing the same, wherein a photoconductive semiconductor thin film is deposited on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. However, the principles of large area film deposition are limited to cylindrical substrates for electrophotographic photoreceptors, and the teachings described therein are not directly transferable to planar substrates of large area. Further, the film-forming process is to be practiced in a batch system and the amount of film products obtained by one batch is limited. The specification does not teach anything about continuous film deposition on a large area planar substrate.
Now, there are still left various problems to be solved for large area film deposition by the MW-PCVD method because non-uniformity of a microwave energy is apt to occur in microwave plasma due to the wavelength of a microwave being short. For instance, in this respect, there is an attempt to use a slow microwave structure in order to provide uniformity of the microwave energy. However, there is an inherent problem in the slow microwave structure, that is the very rapid fall-off of microwave coupling into the plasma as a function of distance transverse to the microwave applicator. In order to solve this problem, it has been proposed that the spacing of the slow microwave structure from a substrate to be processed be varied to thereby make the energy density at the surface of the substrate uniform along the direction of movement of the substrate.
For example, such proposal can be found in the specification of U.S. Pat. No. 3,814,983 or the specification of U.S. Pat. No. 4,521,717. More particularly, the former patent specification discloses that it is necessary to incline the slow wave structure at a certain angle with respect to the substrate. However, inclination of the slow wave structure reversely leads to an insufficient coupling of a microwave energy into the plasma.
The latter patent specification discloses the use of two slow wave structures in an anti-parallel arrangement but in parallel to the substrate. More particularly, the latter patent specification discloses: that it is desired to set the two slow wave applicators at an angle to each other that the planes normal to the medians of the applicators intersect at a straight line which extends parallel to the surfaces of the substrate to be processed and at right angles to the travel direction of the substrate; and that in order to avoid structive interference between the two slow wave applicators, it is desired to displace the two slow wave applicators from each other traversely of the travel direction of the substrate by a distance equal to half of the space between the crossbars of the waveguide.
Several proposals have been made in order to provide plasma uniformity and more particularly, energy uniformity as found, for example, in J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 126-130 and pp. 295-298. These reports describe a microwave reactor called a microwave plasma disc source (MPDS) and that the plasma is in the shape of a disc or tablet, with a diameter that is a function of microwave frequency. More particularly, the reports describe that: the plasma disc source can be varied with the frequency of microwave; however, in the case of a microwave plasma disc source which is designed for operation at the normal microwave frequency of 2.45 GHz, the plasma confined diameter is about 10 centimeters at the most and the plasma volume is about 118 cm.sup.3 at the most, thus this is far from a large surface area; in the case of a system designed for operation at the lower frequency of 915 MHz, the lower frequency source would provide a plasma diameter of approximately 40 cm with a plasma volume of 2000 cm.sup.3 ; and the microwave plasma disc source can be scaled up to discharge diameters in excess of 1 cm by operating at still lower frequencies, for example 40 MHz, however extreme expenses are required to establish such an apparatus which can perform this.
In order to effectively provide high density plasma using microwave, there have been proposed methods to establish the electron cyclotron resonance condition (namely, the ECR condition) by arranging electromagnets around the cavity resonator as found in Japanese Unexamined Patent Publications Sho. 55(1980)-141729 and Sho. 57(1982)-133636. And at academic meetings, etc., there have been reported methods of forming various semiconductor thin films by utilizing high density plasma and some microwave ECR plasma CVD apparatus capable of performing such methods have been commercialized.
However, it has been generally recognized in the technical field to which the invention pertains that it is technically difficult to form a deposited film uniformly over a large area substrate because of non-uniformity of plasma caused by the wavelength of microwave and also because of non-uniformity of magnetic field distribution due to the use of the magnets for the control of plasma.
In the case where the microwave ECR plasma CVD apparatus is intended to scale up so that film deposition over a large area can be done, there are such various problems to be solved beforehand that electromagnets to be used are necessary to be also scaled up; means for preventing the apparatus from overheating is necessary to be provided; a special DC high power regulated supply is necessary to be provided; and the like.
Further, the deposited film obtained by the known microwave ECR plasma CVD method is usually inferior to the deposited film obtained by the known RF plasma CVD method with respect to film property. Further, in addition, in the case of forming a deposited film on a substrate by the microwave ECR plasma CVD method, there is a distinguishable difference with respect to the film deposition rate and the film property between the film formed in the space where the ECR condition is established and the film formed in the space where the ECR condition in not established, in the dispersed magnetic field space in other words. In view of this, the microwave ECR plasma CVD method is not suitable for the preparation of a semiconductor device required to excel in quality and in uniformity with respect to the characteristics to be provided.
The foregoing U.S. Pat. Nos. 4,517,223 and 4,729,341 describe the necessity of maintaining very low pressures in order to provide high density plasmas. That is, they describe that the use of low pressures is necessary in order to obtain high film deposition rates and/or high gas utilization efficiency.
However, any of the foregoing slow wave structure and electron cyclotron resonance methods is not sufficient in order to maintain the relationships among high film deposition rate, high gas utilization efficiency, high power density and low pressure.
In view of what is described above, there is an increased demand for eliminating the foregoing problems of the known microwave plasma CVD method and providing an improved microwave plasma CVD process which is free of such problems.
There is also other demand for providing a large area or lengthy thin semiconductor film excelling in quality and uniformity of characteristics which is desirably usable in not only solar cells but also in semiconductor devices such as TFTs, photoelectric conversion elements for contact image sensors, switching elements, image input line sensors, etc. at a reduced cost.